Antiplatelets & AnticoagulantsA. Min Kang, MDMedical Toxicology FellowBanner – University Medical Center Phoenix

Goals

•Review hemostasis and lab testing.•Review common antiplatelet and

anticoagulant agents.•Discuss treatment options in overdose.

Overview

•Primary Hemostasis▫Platelets

•Secondary Hemostasis▫Coagulation Cascades

PRIMARY Hemostasis

Primary Hemostasis (Platelets)GpIIb/IIIa – vWF/fibrinogen – GpIIb/IIIa

Collagen – vWF - GpIb

Activation: ADP receptors

Aspirin

•Irreversibly inhibits COX-1 inhibits TXA2 formation

Aspirin Overdose

•Platelet transfusion•ddAVP may provide some short-term

effect

QuestionA 65 y/o M with CAD and recent cardiac stents presents after reportedly overdosing on his clopidogrel tablets.What is the mechanism of action of clopidogrel?

a) Inhibit platelet adherence by blocking GpIb.b) Inhibit platelet activation/aggregation by

blocking ADP.c) Inhibit platelet aggregation by blocking

GpIIb/IIIa.d) Inhibit platelet aggregation by blocking COX.

ADP-R

P2Y12 Inhibitors

•Purine analogs – reversible antagonists▫Ticagrelor▫Cangrelor

P2Y12 Inhibitors

•Thienopyridines – irreversible antagonists▫All are prodrugs▫All associated with TTP, neutropenia,

aplastic anemia

Clopidogrel

• Prodrug• Pharmacokinetics:

▫Absorption: rapid, >50% oral May be dose-limited

▫Metabolism: 85% hydrolyzed to inactive clopidogrel-carboxylic

acid Cytochromes (?2C19): 2-oxo-clopidogrel

(intermediate) Hydrolyzed or converted to active thiol metabolite

▫Elimination: t1/2 of thiol metabolite = 30 mins

P2Y12 Inhibitor Overdose

•Uncommon▫Platelet transfusion does not appear to

work▫ddAVP partially reverses bleeding time in

rats, improves platelet function assays in humans

Glycoprotein IIb-IIIa Antagonists•Blocks platelet GpIIb/IIIa receptor:

▫Blocks binding of fibrinogen, vWF on activated platelets

▫Inhibits platelet aggregation•Abciximab – chimeric (human-mouse) Fab•Eptifibatide•Tirofiban

SE pygmy rattlesnake(Sistrurus miliarius barbouri)

SECONDARY Hemostasis

Classical Coagulation

Extrinsic Pathway

Intrinsic Pathway

Common Pathway

Fibrin Clot

Question

A 35 y/o M with no PMH presents 2 hours after ingesting a handful of a relative’s warfarin tablets in a suicide attempt. He is asymptomatic and coagulation labs (PT/INR & aPTT) are within normal limits.What is an appropriate next step?a) Clear the patient for psychiatry.b) Treat with Vitamin K now.c) Treat with FFP now.d) Admit and monitor labs for 24 hours.

Warfarin History• In 1920s, livestock in Canadian

prairies and US plains started dying from hemorrhage

•Moldy hay from sweet clover (“sweet clover disease”)

•Coumarin became oxidized to dicoumarol

•Work funded by Wisconsin Alumni Research Foundation (WARF)

Warfarin History•In 1948, Karl Link promoted warfarin as a rodenticide

•President Dwight Eisenhower was given warfarin in 1955

Warfarin

•Absorption:▫Complete GI absorption▫Peak concentration in ~90 mins

•Distribution:▫99% protein bound

•MOA: Vitamin K antagonist•PT is most sensitive since Factor 7 has

shortest half-life

Vitamin K Cycle

Vitamin K Cycle

Inactive

Active

Vitamin K Cycle

Classical Coagulation

Factor 7Tissue Factor

Extrinsic Pathway

Intrinsic Pathway

Common Pathway

Factor 10Factor 5

Factor 2 (Prothrombin)

Fibrin Clot

PT vs. INR•PT:

▫Plasma + Ca2+ + [TF + phospholipids] (thromboplastin)

▫Commercial thromboplastins vary leads to different sensitivities to VKA

• INR:▫WHO proposed INR system in 1983 to

standardize testing▫INR = [PT / mean normal PT]ISI

▫ISI indicates overall sensitivity of the reagent to reductions in factors II, VII, and X

Classical Coagulation

Factor 7Tissue Factor

Extrinsic Pathway

Intrinsic Pathway

Common Pathway

Factor 10Factor 5

Factor 2 (Prothrombin)

Fibrin Clot

PT/INR PT/INR PT/INR

PT/INR

QuestionA 49 y/o M presents to the ED with nausea, vomiting, and bloody stools. He is tachycardic but has normal blood pressure. He reports ingesting d-CON mouse poison on multiple occasions over the past several weeks. Pertinent labs are listed:Hgb = 9.9Hct = 27.5Platelet = 239PT and PTT unmeasurable

Which of the following can correct the coagulopathy?a) Platelet transfusion.b) ddAVP.c) Vitamin K.d) Cryoprecipitate.

Anticoagulant Rodenticides

•1st Generation:▫Warfarin▫Chlorophacinone▫Diphacinone

•2nd Generation:▫Brodifacoum▫Difenacoum▫Bromadiolone▫Difethialone

EPA & Rodenticides• In 2008, EPA tightened safety standards on

household rodenticide products▫Bait stations▫Block or paste poison must be inside the station▫< 1 pound of poison▫Cannot sell second-generation agents to

consumers•Production of many d-CON products ceased

12/31/2014•Distribution to retailers ended 03/31/2015

VKA Reversal• Vitamin K1 (phytonadione) PO or IV• FFP

▫ Time to cross-match and thaw▫ Large volume▫ Infection risk; TRALI (1 in 5,000)

• Prothrombin Complex Concentrates:▫ No cross-match; Small volume; short infusion time▫ 3-Factor Prothrombin Complex Concentrate (Bebulin; Profilnine):

Factors 2, 9, 10▫ 4-Factor Prothrombin Complex Concentrate (Kcentra):

Adds Factor 7▫ Most PCCs have small amount of heparin and AT

Contraindicated in HIT• rh7a• ACCP recommends 4-factor PCC over FFP (Grade 2C)

4-Factor PCC (Kcentra)

•FDA approved for VKA-induced major bleeding or need for urgent surgery/invasive procedure

•~500 or ~1,000 unit vials•Contains:

▫Factors 2, 7, 9, 10▫Protein C, S▫Heparin, AT3

VKA Reversal

•Factor Eight Inhibitor Bypassing Activity (FEIBA):▫AKA Anti-Inhibitor Coagulant Complex▫Indications: Prophylaxis and bleeding in

Hemophilia A/B with inhibitors▫Pooled plasma

Factors 2, 9, 10 (non-activated) Factor 7 (activated) Factor 8

Question

A 50 y/o M presented to the ED after reportedly injecting himself subcutaneously with 1,500 mg (17 mg/kg) of enoxaparin 3 hours prior.Which of the following agents may be helpful if the patient begins to have major bleeding?a) Vitamin K.b) Platelet transfusion.c) Cryoprecipitate.d) Protamine.

Heparins

•Bind antithrombin-3•Heparin:

▫Inhibits Factor 10a (depends on AT3)▫Inhibits Factor 2a

•LMWH: Inhibits Factor 10a > Factor 2a▫Enoxaparin, dalteparin, tinzaparin

Coagulation Tests

•aPTT:▫Plasma + Ca2+ + [phospholipids] (partial

thromboplastin)▫Tests Intrinsic Pathway

•Thrombin Time:▫Platelet-poor plasma + thrombin

Anti-Factor 10a Assay•Sample should be drawn at peak plasma

concentration (~4 hrs post-dose for LMWH)•Test Components:

▫Synthetic Factor 10a substrate with chromophore

▫Patient’s Factor 10a can cleave the chromophore (if not inhibited by an anticoagulant)

▫Less color change means more anticoagulant

Anti-Factor 10a Activity

•Compare to a standard specific for the anticoagulant

•Results reported as drug concentration (units/mL)

Bates & Weitz (2005). Circulation 112: e53-60.

Classical Coagulation

Factor 7Tissue Factor

Factors 9, 11, 12Factor 8

Extrinsic Pathway

Intrinsic Pathway

Common Pathway

Factor 10Factor 5

Fibrin Clot

aPTT

PT/INRaPTTAnti-Factor 10a

PT/INRaPTT

PT/INRaPTTTT

Factor 2 (Prothrombin)

PT/INR

Protamine•Derived from fish sperm.•Fully reverses heparin:

▫1 mg protamine for each 100 units heparin•Neutralizes anti-2a activity of LMWH:

▫Will reverse aPTT and TT▫Only 60% of anti-Factor 10a activity is reversed

• Increased risk for anaphylaxis:▫Previous use of protamine-sulfate insulin▫Vasectomy▫Fish sensitivity

Question

A 36 y/o M presents after a reported ingestion of a handful of rivaroxaban ~1.5 hr prior to arrival. (He does not take it chronically.)Which lab test would most likely help determine if he is telling the truth?

a) PTb) aPTTc) ECTd) TT

Direct Factor 10a Inhibitors

•Inhibit bound and free Factor Xa•Does not depend on AT3

Classical Coagulation:Factor 10a Inhibitor

Factor 7Tissue Factor

Factors 12, 11, 9Factor 8

Extrinsic Pathway

Intrinsic Pathway

Common Pathway

Factor 10Factor 5

Fibrin ClotX

Factor 2 (Prothrombin)

Indications

•Nonvalvular atrial fibrillation•Deep vein thrombosis (DVT) & pulmonary

embolism (PE) treatment•DVT prophylaxis after hip/knee

replacement (rivaroxaban, apixaban)

Rivaroxaban and PT

Arachchillage et al (2014). Thromb Haemost 112: 421-3.

Apixaban and PT/aPTT

PT

aPTT

Dale et al (2014). J Thromb Haemost 12: 1810-5.

Testing for 10a Inhibitors

Medication

PT aPTT Anti-Factor 10a Assay

rivaroxaban Sensitivity

depends on thromboplastin reagent

Less sensitive than PT Increased (need

standard for calibration)

apixaban Minimal effect

edoxaban Less sensitive than PT

ECT & TT are not affected.

Question

A 65 y/o M taking rivaroxaban for atrial fibrillation presents with a lower GI bleed.Which of the following can reverse the coagulopathy of rivaroxaban?

a) Vitamin K.b) Platelet transfusion.c) Cryoprecipitate.d) 4-factor PCC.

Factor Xa Inhibitor Overdose

•Activated charcoal•HD not useful•4-factor PCC (Eerenberg et al, 2011)•FEIBA•rhF7a•3-factor PCC•Andexanet alfa

▫Inactive Factor 10a mimic▫Andexanet Video

Question

A 35 y/o F overdosed on her bottle of dabigatran. She is brought to the hospital with active GI bleeding and is noted to be hypotensive and tachycardic. Which of the following would have the highest potential of reversing this coagulopathy?a) Vitamin Kb) Plateletsc) FFPd) FEIBA

Thrombin & Direct Thrombin Inhibitors (DTIs)

•Thrombin▫Converts fibrinogen

fibrin▫Activates platelets

Dabigatran(Pradaxa)

• Pharmacokinetics:▫Absorption: 3-7% PO bioavailability▫Distribution: 35% protein binding▫Metabolism:

Prodrug quickly & completely hydrolyzed by serum esterases to active dabigatran

20% conjugated to active glucuronides▫Elimination:

80% renal t1/2 = 12-17 hrs; prolonged with renal insufficiency

• Testing:▫TT most sensitive, followed by ECT and aPTT

ECT (Ecarin Clotting Time)•Ecarin is metalloprotease from saw-scaled

viper (Echis carinatus) venom•Only affected by DTIs

▫Not affected by heparin, warfarin, LA

Favaloro et al (2011). Pathology 43: 682-92.

Dabigatran Lab Tests

PT aPTT Anti-10a

TT ECT

Sensitivity depends on assay

Prolonged but non-linear and can underestimate

No effect

Most sensitive

Sensitive but no standard for calibration

- HEMOCLOT Thrombin Inhibitor- Modified TT assay- Not affected by low fibrinogen levels (sample is mixed

with normal plasma)- Standards for hirudin, argatroban, dabigatran

Dabigatran Overdose• In vitro: Activated charcoal binds• Consider HD (~2/3 removed) but rebound

documented• FEIBA

▫Reversed some coagulation tests in vitro▫Decreased bleeding time in rat tail model

• 4-factor PCC▫Did not reverse coagulation tests in vivo (Eerenberg

et al 2011)▫Decreased bleeding in rabbit model

• No evidence for desmopressin, tranexamic acid, aminocaproic acid

Dabigatran Overdose: PER977

•Idarucizumab•PER977:

▫Binds heparin, LMWH, direct factor 10a inhibitors, dabigatran (DTI)

Summary: Classical Coagulation

Factor 7Tissue Factor

Factors 9, 11, 12Factor 8

Extrinsic Pathway

Intrinsic Pathway

Common Pathway

Factor 10Factor 5

Fibrinogen &

Fibrin Clot

aPTT

PT/INRaPTTAnti-Factor 10a

PT/INRaPTTECT

PT/INRaPTTECTTT

Factor 2 (Prothrombin)

PT/INR

Summary: TestingClass Prolonged Normal Overdose

Treatment

VKA PT/INR, aPTT ECT, TT Vitamin K, PCC-4, FFP

Heparins aPTT; Anti-Factor Xa; TTPT may change with extreme UFH doses

ECT Protamine(does not fully reverse anti-10a activity of LMWH)

Direct Factor Xa Inhibitors

PT; Anti-Factor XaaPTT may change but less sensitive

ECT; TT CharcoalPCC-4Andexanet

Direct Thrombin Inhibitors

TT > ECT > aPTTPT may change depending on assay

Anti-Factor Xa

CharcoalHDFEIBAIdarucizumab

If you are in Arizona and have an out-of-state cell phone, dial (602) 253-3334 to reach the poison center.

References• Ageno et al (2012). Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed:

American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 141: e44S-88S.• Ansell et al (2014). Use of PER977 to reverse the anticoagulant effect of edoxaban. N Engl J Med 371: 2141-2.• Arachchillage et al (2014). Comparative sensitivity of commonly used thromboplastins to ex vivo therapeutic

rivaroxaban levels. Thromb haemost 112: 421-3.• Cuisset et al (2010). Clopidogrel resistance: what's new? Arch Cardiovasc Dis 103: 349-53.• Eerenberg et al (2011). Reversal of rivaroxaban and dabigatran by prothrombin complex concentrate: a

randomized, placebo-controlled, crossover study in healthy subjects. Circulation 124: 1573-9.• Garcia et al (2012). Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed:

American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 141: e24S-43S• Gehrie & Laposata (2012). Test of the month: The chromogenic antifactor Xa assay. Am J Hematol 87: 194-6.• Godier et al (2015). Inefficacy of platelet transfusion to reverse ticagrelor. N Engl J Med 372: 196-7.• Hankey & Eikelboom (2011). Dabigatran etexilate: a new oral thrombin inhibitor. Circulation 123: 1436-50.• Holbrook et al (2012). Evidence-based management of anticoagulant therapy: Antithrombotic Therapy and

Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 141: e152S-84S.

• Lindhoff-Last et al (2010). Assays for measuring rivaroxaban: their suitability and limitations. Ther Drug Monit 32: 673-9.

• Morishima & Kamisato (2015). Laboratory measurements of the oral direct factor Xa inhibitor edoxaban: comparison of prothrombin time, activated partial thromboplastin time, and thrombin generation assay. Am J Clin Pathol 143: 241-7.

• Pollack et al (2015). Idarucizumab for Dabigatran Reversal. N Engl J Med.• Taylor et al (2013). Is platelet transfusion efficient to restore platelet reactivity in patients who are responders to

aspirin and/or clopidogrel before emergency surgery? J Trauma Acute Care Surg 74: 1367-9.• Wardrop & Keeling (2008). The story of the discovery of heparin and warfarin. Br J Haematol 141: 757-63.